Very lousy plastic, space bar generated clunk, clunk sound, but otherwise nice wireless keyboard if you don't care much about quality of plastic.
Protocol documentation
• logitech_hidpp_2.0_specification_draft_2012‑06‑04.pdf  from http://6xq.net/git/lars/lshidpp.git/plain/doc/logitech_hidpp_2.0_specification_draft_2012‑06‑04.pdf 
Linux software
• https://github.com/pwr/Solaar 
dpavlin@blue:/rest/cvs/Solaar$ sudo ./bin/solaar show
Unifying Receiver
  Device path  : /dev/hidraw3
  USB id       : 046d:c52b
  Serial       : 656F91A9
    Firmware   : 12.01.B0019
    Bootloader : 02.14
  Has 1 paired device(s) out of a maximum of 6.
  Notifications: (none)
  Device activity counters: 1=2

  1: Wireless Touch Keyboard K400
     Codename     : K400
     Kind         : keyboard
     Wireless PID : 4024
     Protocol     : HID++ 2.0
     Polling rate : 8 ms (125Hz)
     Serial number: AC5F5069
          Firmware: RQK 50.01.B0013
     The power switch is located on the edge of top right corner.
     Supports 22 HID++ 2.0 features:
         0: ROOT                   {0000}   
         1: FEATURE SET            {0001}   
         2: FEATURE INFO           {0002}   
         3: DEVICE FW VERSION      {0003}   
         4: DEVICE NAME            {0005}   
         5: BATTERY STATUS         {1000}   
         6: REPROG CONTROLS        {1B00}   
         7: REPROG CONTROLS V3     {1B03}   
         8: WIRELESS DEVICE STATUS {1D4B}   
         9: unknown:1DF0           {1DF0}   hidden
        10: unknown:1DF3           {1DF3}   internal, hidden
        11: unknown:1E00           {1E00}   hidden
        12: VERTICAL SCROLLING     {2100}   
        13: HI RES SCROLLING       {2120}   
        14: MOUSE POINTER          {2200}   
        15: FN INVERSION           {40A0}   
        16: NEW FN INVERSION       {40A2}   
        17: ENCRYPTION             {4100}   
        18: KEYBOARD LAYOUT        {4520}   
        19: TOUCHPAD FW ITEMS      {6010}   
        20: TOUCHPAD SW ITEMS      {6011}   
        21: TOUCHPAD WIN8 FW ITEMS {6012}   
     Has 18 reprogrammable keys:
         0: SHOW DESKTOP               => ShowDesktop                   is FN, reprogrammable
         1: METRO APPSWITCH            => MetroAppSwitch                is FN, reprogrammable
         2: METRO SEARCH               => MetroSearch                   is FN, reprogrammable
         3: METRO SHARE                => MetroShare                    is FN, reprogrammable
         4: METRO DEVICES              => MetroDevices                  is FN, reprogrammable
         5: METRO SETTINGS             => MetroSettings                 is FN, reprogrammable
         6: Previous                   => Previous                      is FN
         7: Play/Pause                 => Play/Pause                    is FN
         8: Next                       => Next                          is FN
         9: Mute                       => Mute                          is FN
        10: Volume Down                => Volume Down                   is FN
        11: Volume Up                  => Volume Up                     is FN
        12: MEDIA PLAYER               => Music                         nonstandard, reprogrammable
        13: Lock PC                    => WindowsLock                   nonstandard, reprogrammable
        14: SLEEP                      => Sleep                         nonstandard, reprogrammable
        15: LEFT CLICK                 => LeftClick                     mse
        16: LEFT CLICK                 => LeftClick                     mse
        17: RIGHT CLICK                => RightClick                    mse
     Battery: 90%, discharging.

Firmware update under Linux
• https://github.com/hughsie/fwupd 
• https://blogs.gnome.org/hughsie/2017/05/22/updating‑logitech‑hardware‑on‑linux/